System and Method for Seamless Integration of CWDM and DWDM Technologies on a Fiber Optics Infrastructure

ABSTRACT

The present invention provides a system and method for multiplexing DWDM channels on top existing CWDM infrastructure. An erbium-doped fiber amplifier amplifies DWDM signals in the DWDM domain to compensate for 10 G optics power budget limitations without blocking the CWDM signals. A passive WDM infrastructure allows the CWDM and DWDM signals to be multiplexed and de-multiplexed on the same fiber and allows seamless integration with existing infrastructure avoiding the need to sacrifice CWDM channels.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to optical networking and morespecifically, to a system and method for integrating CWDM and DWDMtechnologies on a common fiber optics infrastructure.

2. Description of Related Art

Coarse wavelength division multiplexing (CWDM) is a method of combiningmultiple signals on laser beams at various wavelengths for transmissionalong fiber optic cables. CWDM systems are a popular choice for metroaccess networks and major telecoms have a significant capital investmentin the CWDM infrastructure. Although the number of channels in a CWDMsystem is fewer than in a dense wavelength division multiplexing (DWDM)system, CWDM remains widely deployed in metro access networks where thedistance is limited to about 80 kilometers (km). As used herein, CWDMrefers to an ITU (International Telecommunications Union) standard whichincludes the specification of the particular channel wavelengths and thespacing between these channels. DWDM refers to an ITU standard in whichthe channel spacing is tighter so more wavelength channels are packedinto an optical fiber.

With the continued growth in network traffic, telecoms are motivated toupgrade the capacity of their network to meet customer expectations.This means that telecoms need to increase channel density by addingadditional wavelengths. However, CWDM is effectively limited to abouteight different wavelengths on common ITU-T G.652 fiber (type A and B).The G.652a and G.652b specifications define the optical fiberspecifications. These optical fibers are typically found in extendedlength LAN, MAN and access network systems. Clearly, rather than rippingout the CWDM network and replacing it with a DWDM network, telecoms needa cost effective solution that can increase channel density by addingwavelengths in a seamless, non-evasive manner to the CWDM network.

On approach that has been suggested is to cannibalize a portion of theCWDM wavelengths to route DWDM channels. Because, DWDM has tighterchannel spacing, replacing 25% to 50% of the CWDM channels with DWDMchannels results in an overall increase in channel density.Unfortunately, this approach has several shortcomings. For example, thetelecom loses a significant portion of their CWDM bandwidth, which isclearly undesirable. Further, because of the optical characteristics ofthe 1 GbE and 10 GbE, performance is degraded and the network is limitedto much less than 80 kilometers (km). Thus, the telecom would have toredesign the entire network to take into account the degradedperformance. Further still, the DWDM channels undergo significantly moreattenuation than the CWDM channels which is a critical limitation for 10G application already suffering a power budget gap with ½ GbE CWDMchannels.

Notwithstanding the problems with adding addition channels, telecoms arealso motivated to upgrade the data rates of their network to meetcustomer expectations. This means that telecoms need to increase datarates on at least part of the channel capacity. Since most of theinstalled CWDM networks already include the technology infrastructure tosupport 1 Gigabit Ethernet (GbE), the natural progression would be toupgrade the CWDM infrastructure to handle 10 GbE. However, because DWDMtechnology dominates the 10 GbE market, there is only limited marketopportunity for 10 GbE CWDM technology and the acquisition price forthat technology too high. Thus, telecoms are being forced to upgradetheir infrastructure to 10 GbE DWDM.

The upgrade to 10 GbE DWDM means that telecoms either have to string newfiber or mix DWDM with CWDM optical technologies on the same fiber.Unfortunately, because the 10 G optics has a reduced power budgetcompared to the lower speed GbE optics, it is not possible to simplyinsert 10 G optics on existing CWDM installations because any opticalamplification of the DWDM would block the CWDM wavelengths that areoutside the pass band of the amplifiers.

Accordingly, most telecoms resort to leaving the traditional CWDMnetwork intact and stringing a separate fiber to handle the DWDM networktraffic. Not only is this an expensive alternative, laying new fiber isintrusive and potentially disruptive to the existing network as newpower supplies and other infrastructure is swapped out to handle the newnetwork.

What is needed is a system and method that increases the channel densityof CWDM networks and migrates the CWDM networks to 10 GbE in a seamlessand non-invasive manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary and simplified CWDM optical network 10that includes both CWDM and DWDM channels in accordance with anembodiment of the present invention.

FIG. 2 illustrates the typical channel spacing in a CWDM system togetherwith the DWDM channel overlay in accordance with an embodiment of thepresent invention.

FIG. 3 illustrates a DWDM multiplexer for multiplexing DWDM channelsonto the CWDM network in accordance with an embodiment of the presentinvention.

FIG. 4 illustrates the receiver portion of demultiplexer for recoveringDWDM channels from the CWDM network in accordance with an embodiment ofthe present invention.

FIG. 5 illustrates a transceiver having a DWDM multiplexer and a DWDMdemultiplexer in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to optical networking and morespecifically, to a system and method for integrating CWDM and DWDMtechnologies on a common fiber optics infrastructure that provides anincreases channel density and migrates current lower speed GbE/OC-48CWDM infrastructure to 10 GbE in a seamless and non-invasive manner.

In the following description of embodiments of the present invention,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific embodiments inwhich the invention may be practiced. It is to be understood that otherembodiments may be utilized and that changes may be made withoutdeparting from the scope of the present invention.

Further, in the following description of embodiments of the presentinvention, numerous specific details are presented to provide a completeunderstanding of the embodiments of the invention. One skilled in therelevant art will recognize, however, that the invention may bepracticed without one or more of the specific details, or with othermethods, components, etc. In other instances, well-known structures oroperations are not shown or described in detail to avoid obscuringaspects of various embodiments of the invention. Wherever possible, thesame reference numbers will be used throughout in the drawings to referto the same or like components.

Refer now to FIG. 1, which illustrates an exemplary and simplified CWDMoptical network 10 that includes both CWDM and DWDM channels. Morespecifically, optical network 10 includes an optical multiplexer 12receiving a plurality of CWDM channels, generally indicated at 13. Inthe illustrated embodiment, the channels have wavelengths spaced 20nanometers (nm) apart at eight defined wavelengths: 1611 nm, 1591 nm,1571 nm, 1551 nm, 1531 nm, 1511 nm, 1491 nm, and 1471 nm. As is wellunderstood, the energy from the lasers in a CWDM system is spread outover a larger range of wavelengths than is the energy from the lasers ina DWDM system. The tolerance or extent of wavelength imprecision orvariability in a CWDM laser is much looser compared to the tightertolerance required of a DWDM laser. Because of the use of lasers withlower precision, a CWDM system is less expensive and consumes less powerthan a DWDM system.

The output of multiplexer 12 is routed to a DWDM multiplexer 14 wherethe CWDM signal is multiplexed onto fiber 15 with a plurality of DWDMsignals, generally indicated at 16. Advantageously, none of the CWDMchannels are sacrificed to provide the plurality of DWDM channels.

Before the DWDM signals are multiplexed with the CWDM signals, they areamplified in the DWDM domain by erbium doped amplifiers 17 to compensatefor the reduced power budget of 10 G optics.

The combined CWDM and DWDM signals are demultiplexed by DWDMdemultiplexer 20 that separates the DWDM from the CWDM signals. The DWDMsignals are provided as outputs as generally indicated at 21. The CWDMsignal is routed to a second demultiplexer, CWDM demultiplexer 22 thatprovides the plurality of CWDM signals, as generally indicated at 23.

FIG. 2 illustrates the typical channel spacing in a CWDM system togetherwith the DWDM channel overlay. As is well understood in the art, commonCWDM filter design calls for eight channels aligned on the ITU-T G.694.1grid (1471, 1491, 1511, 1531, 1551, 1571, 1591, 1611 nm). One skilled inthe art will understand that other channel alignments are feasible (forexample, the grid may begin at 1470 with a channel spacing of 20 nm.Further, there may be more than eight CWDM channels depending on thespecific implementation of a network.

Each CWDM filter bandwidth is about 12 nm wide. This means that the 1551nm filter channel extends as high as 1557 nm, as indicated at 25. The1571 nm filter channel extends as low as 1565 nm, as indicated at 26.Similarly, the 1531 channels extends up to 1537 nm, as indicated at 27,while the 1551 channels extends down to 1545 nm, as indicated at 28.

The CWDM filter design leaves two spectral regions, in between CWDMchannels, that overlap with the C-band where DWDM wavelengths reside.The C-band, or conventional band, refers to the spectral window fromabout 1525 nm to 1565 nm. This window also corresponds to the amplifyingrange of erbium-doped fiber amplifiers 17. With the ITU-T G.692 DWDMgrid and appropriate CWDM passive filters, eight additional DWDMchannels are inserted in between existing CWDM channels. In oneembodiment, additional band filters are used to provide about 30 dBisolation between CWDM and DWDM channels.

The eight additional DWDM channels are divided into two groups. Onegroup in the 1538.98 to 1542.94 spectral window is inserted between the1531 and 1551 CWDM channels as indicated at 29. The second group in the1548.98 to 1560.61 spectral window are inserted between the 1551 and the1570 CWDM channels as indicated at 30.

FIG. 3 illustrates DWDM multiplexer 14 in greater detail. Specifically,DWDM 14 comprises a plurality of DWDM inputs, indicated generally at 31.In one embodiment, two groups of DWDM channels are defined, one grouphaving three wavelength and one group having five wavelengths for atotal number of eight DWDM channels in addition to eight CWDM channelsreceived from CWDM multiplexer 12. The multiplexed CWDM channels arereceived on input 32.

Each of the two groups of DWDM signals are passed through an appropriatechannel filter, indicated generally at 33, and combined. The combinedchannels from each group are then routed to an edge filter 35 to ensureadequate isolation from the CWDM signals.

The output of edge filter 35 is routed to erbium doped amplifiers 17 tocompensate for the reduced power budget of 10 G optics. The presentinvention deploys amplification on the 10 G DWDM channels alone becausethese channels are the ones short of power budget. The challenge is todeploy optical amplification in a manner that does not affect the CWDMchannels because most optical amplifiers will cut-off any wavelengthoutside their amplification range (C-band for common metro devices).Because it is not possible to pass CWDM wavelengths through DWDMamplifiers, it is not possible to place an amplifier on the fiber whereboth CWDM and DWDM channels co-exist. The addition of amplifiers 17advantageously amplifies the DWDM channels before combination with theCWDM signals.

To illustrate, consider that a typical CWDM 1 GbE packaged in a SmallForm factor Pluggable (SFP) that has a 29 dB power budget over 80 km andassume that the dispersion penalty is negligible. The maximum fiber losswill be 29 dB-4.4 dB (which is the worst case loss through themultiplexer 12 and demultiplexer 22)-2.2 dB (DWDM filter pass-through inDWDM multiplexer 14)=22.4 dB

10GE DWDM Xenpak has 20 dB of power budget over 80 km with approximately3 dB of dispersion penalty. The maximum fiber loss is thus 20 dB-4.7 dB(DWDM multiplexer 14 and demultiplexer 20)=15.3 dB. Thus, there is a 7.1dB difference in fiber budget which means approximately 25 to 30 km lessdistance for the 10 GE channels.

Amplifier 17 may be a mini-erbium-doped fiber amplifier (EDFA), which isa very low cost optical device with several meters of glass fiber dopedwith erbium ions that boosts an optical signal when the erbium ions areexcited to a high energy state or an erbium-doped waveguide amplifier(EDWA) which is an optical amplifier similar to an EDFA, but whichderives a higher gain through a small waveguide rather than severalmeters of fiber. Amplifier is put on the MUX path to bump up the powerof the eight 10 GbE DWDM channels by about +7 dB.

The enclosure housing multiplexer 14 includes two additional customeraccessible ports to couple the DWDM signals to optical amplifier 17.Amplifier 17 amplifies the DWDM channels before they get multiplexedwith the CWDM signals on the fiber trunk.

In this manner, amplifier 17 provides enough power to the eight DWDM 10GbE channels overlaying the CWDM cloud to transport the DWDM 10 GbEchannels at least 80 km. This eliminates the need to develop special 10G CWDM lasers. Further, there is no need to sacrifice any of theexisting CWDM channels when the DWDM 10 GbE channels are added to thesame fiber infrastructure.

The output of amplifier 17 is routed to a second edge filter 36 tore-shape the DWDM signals. Filter 36 feeds one of the two groups, forexample channels 21-23, to a band filter 37 where the DWDM aremultiplexed onto the CWDM signal (from input 32) as it is reflected byfilter 37. The combined signal is then reflected at band filter 38 wherethe second of the two groups of DWDM channels are multiplexed onto thecombined signal. The output of filter 38 is sent out of output port 39to the receiver along fiber 15.

FIG. 4 illustrates the receiver portion of demultiplexer 20. Morespecifically, the combined CWDM and DWDM signal is received from fiber15 at input port 42. The combined signal is reflected at a first bandfilter 43 where the second of the two DWDM groups are demultiplexed fromthe combined signal. The demultiplexed signal is reflected through aseries of band pass filters, indicated generally at 44, to recover eachof the five channels CH43 (1542.94), CH44 (1542.14), CH46 (1540.56),CH47 (1539.77) and CH48 (1538.98).

The second group of DWDM signals are then reflected at a second bandpass filter 45 where channels CH21 (1560.61), CH22 (1559.79) and CH23(1558.98) are filtered from the combined signal. Channels 21-23 are thenseparated by band pass filters, indicated generally at 46. The DWDMchannels are provided as outputs at output ports 47. The CWDM signal isthen routed through two additional band filters, indicated generally at48, to further improve isolation and then from output port 49 to theCWDM de-multiplexer 22 (see FIG. 1) where CWDM signals 1471-1611 arerecovered.

In one embodiment of the present invention, a separate DWDM multiplexerand amplifier are provided on the transmit side of network 15 and aseparate DWDM de-multiplexer is provided on the receive side of network15. In another embodiment, each side of network 15 comprises a modulethat includes the DWDM multiplexer, amplifier, and DWDM de-multiplexerin a single enclosure.

Advantageously, the present invention increases channel capacity in aCWDM networks without sacrificing existing wavelengths. From thetelecom's perspective, the present invention does not require anynetwork re-engineering because the module is simply cascaded with anexisting CWDM filter.

FIG. 5 illustrates an alternative embodiment for a module 50 thatimplements two additional DWDM 10 GbE channels in a CWDM network. Thislow cost embodiment, includes input port 51 for receiving the CWDMsignals and input ports 52 for receiving DWDM channels. The DWDMchannels are combined by filtering channel 22 with pass band filter 53and then reflecting channel 22 at pass band filter 54 where it iscombined with channel 21. Amplifier 17 then amplifies the DWDM channelsbefore routing the signals to band pass filter 55. The DWDM channels arecombined with the CWDM channel when it is reflected by filter 55. Thecombined DWDM and CWDM signals are then routed to the network 15 throughport 56. Advantageously, there is no requirement for edge filters due tothe low density of DWDM channels.

On the receive side, the combined DWDM and CWDM signal is received onport 58 with the CWDM signal reflected by band filter 59 where the DWDMsignals are removed. The CWDM signal is then reflected by band filter 60to improve isolation before being routed to the CWDM de-multiplexer 22(see FIG. 1) by way of output port 61. The individual DWDM signals,channel 22 and channel 21, are recovered by filter 62 and 63,respectively. Channels 22 and 21 are then available at output ports 64.

In other embodiments, a transceiver 50 that implements four additionalDWDM 10 GbE channels in a CWDM network. In yet another embodiment, atransceiver 50 that implements six additional DWDM 10 GbE channels in aCWDM network. In yet another embodiment, a plurality of additional DWDM10 GbE channels are provided in the CWDM network.

The present invention solves the problem of growing existing CWDMnetworks to 10 GbE using DWDM 10G optics. By amplifying the DWDM signalsin the DWDM domain, the present invention addresses the reduced powerbudget of 10 G optics in a fashion that is totally transparent to theCWDM network.

Therefore, while the description above provides a full and completedisclosure of the preferred embodiments of the present invention,various modifications, alternate constructions, and equivalents will beobvious to those with skill in the art. Thus, the scope of the presentinvention is limited solely by the metes and bounds of the appendedclaims.

1. An optical network comprising: a CWDM multiplexer having at leasteight CWDM signal inputs and a signal output; an amplifier; and a DWDMmultiplexer having a plurality of DWDM signal inputs that are routed tothe amplifier, an input, coupled to the signal output of the CWDMmultiplexer, for receiving a CWDM signal, and an output for a combinedDWDM and CWDM signal.
 2. The optical network of claim 1 wherein saidoptical network is a metro access network.
 3. The optical network ofclaim 2 wherein the optical network spans a distance of about 80kilometers.
 4. The optical network of claim 1 further comprising atleast two 10 GbE DWDM channels.
 5. The optical network of claim 1wherein the DWDM signals are amplified by about 7 dB.
 6. The opticalnetwork of claim 1 wherein the DWDM signals are amplified beforeoverlaying the CWDM signals with sufficient power to transport the DWDM10 GbE channels at least 80 kilometers (km).
 7. The optical network ofclaim 1 wherein the DWDM signals are interspersed between adjacent CWDMchannels.
 8. The optical network of claim 1 wherein a first plurality ofDWDM signals is interspersed between the 1531 and the 1551 CWDMchannels.
 9. The optical network of claim 8 wherein a second pluralityof DWDM signals is interspersed between the 1551 and the 1571 CWDMchannels.
 10. The optical network of claim 1 wherein a plurality of DWDMsignals is interspersed between the 1531 and the 1551 CWDM channels. 11.The optical network of claim 10 wherein the plurality of DWDM signalscomprise up to five DWDM signals.
 12. The optical network of claim 1wherein a plurality of DWDM signals is interspersed between the 1551 andthe 1571 CWDM channels.
 13. The optical network of claim 12 wherein theplurality of DWDM signals comprise up to three DWDM signals.
 14. Anoptical network comprising: a DWDM de-multiplexer having an inputcoupled to the single fiber network and adapted to recovering the DWDMsignals from an optical signal having DWDM signals and CWDM signals. 15.The optical network of claim 14 further comprising: a CWDMde-multiplexer having an input coupled to the DWDM de-multiplexer andadapted to recovering the CWDM signals.
 16. A method for combining 10GbE DWDM on top of a CDWM network infrastructure, the method comprising:filtering a plurality of DWDM signals; amplifying the plurality of DWDMsignals; filtering the plurality of DWDM signals; and multiplexing DWDMchannels on top of an existing CWDM infrastructure.
 17. The method ofclaim 16 wherein the filtering steps comprise passing the DWDM signalsthrough an edge filter.
 18. The method of claim 17 wherein themultiplexing step further comprises: reflecting a CWDM signal at a firstband filter to combine the CWDM signal with a first group of theplurality of DWDM signals; and reflecting the CWDM signal at a secondband filter to combine the CWDM signal with a second group of theplurality of DWDM signals.
 19. The method of claim 18 wherein thecombined CWDM signal and DWDM signal comprises at least eight CWDMchannels and up to eight DWDM channels.
 20. The method of claim 16wherein the DWDM signals are amplified to provide enough of a powerbudget to transmit the DWDM signals at least 80 kilometers(km).
 21. In anetwork comprising 10 GbE DWDM on top of a CDWM network infrastructure,a method comprising: de-multiplexing a second group of the plurality ofDWDM signals; de-multiplexing a first group of the plurality of DWDMsignals; and de-multiplexing a CWDM signals.
 22. The method of claim 25wherein the first group comprises up to five DWDM channels that areinterspersed between two CWDM channels in the C-band.
 23. The method ofclaim 25 wherein the second group comprises up to three DWDM channelsthat are interspersed between two CWDM channels in the C-band.
 24. Themethod of claim 27 further comprising filtering the CWDM channel toprovide at least 30 dB isolation between CWDM and DWDM channels.
 25. Themethod of claim 20 wherein the CWDM signals comprise either 1 or 2Gigabyte Ethernet (GbE) signals and the DWDM signals comprise 10 GbEsignals.
 26. A passive WDM infrastructure for transmitting DWDM signalsover a 1 or 2 Gigabyte Ethernet (GbE) single fiber network comprising:an erbium-doped fiber amplifier for amplifying a plurality of combinedDWDM signals in the DWDM domain to compensate for 10 G optics powerbudget limitations without blocking the CWDM signals; and means formultiplexing CWDM and DWDM signals on the single fiber.